Design und Charakteristik eines satellitentauglichen optisch-parametrischen Oszillators im nahen Infrarot
Aachen (2018) [Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (IX,147 Seiten) : Illustrationen, Diagramme
Within the framework of the Franco-German climate mission MERLIN, the methane concentration in the earth's atmosphere is to be measured with global coverage in order to verify and improve existing climate models. The great advantage of the spaceborne LIDAR measurement system from MERLIN over existing systems is the active measurement process, that enables measurements even without external light sources such as sun and stars and through thin clouds. In the IPDA LIDAR process, two laser pulses are emitted shortly after each other. The first of these pulses is at the online wavelength, an absorption line of methane at 1645,55 nm. The second pulse is at 1645,85 nm as at this wavelength absorption in the atmosphere is especially low. The methane concentration in the measuring column is determined from the difference of the backscattered signals. So far, there are no laser beam sources that meet the requirements at the specified wavelengths, especially with regard to the required pulse parameters (9 mJ, 15 ns, 2x20 Hz), the beam quality as well as the spectral width and purity. Therefore, in MERLIN a Nd:YAG oscillator-amplifier system with an optical parametric oscillator (OPO) for frequency conversion is used. Some of the optical output parameters of the OPO have already been shown, e.g. as part of the airborne CHARM-F mission of DLR-IPA. For use on a satellite, however, additional requirements apply to the conversion efficiency and mechanical and thermal stability. Fraunhofer ILT has therefore developed an innovative assembly technology with soldered optical mounts that is suitable for use in demanding environments. Within the scope of this work, an alignment strategy is developed with which an OPO is assembled, that is based on the new assembly technology and fulfils the optical and mechanical requirements for spaceborne methane measurements. The challenge results from the fact that the use of the new assembly technology significantly limits the degrees of freedom of adjustment when setting up a beam source. First, the optical design of the OPO is optimized using a numerical simulation. Subsequently, an experimental setup, which is operated at the determined working point, shows that the requirements for the target values of the optical output parameters are fulfilled. The numerical simulation is extended by the adjustment variables of the OPO and verified with regard to the reliable prediction of adjustment influences. With the help of the verified simulation, the influences of all adjustment variables on the conversion efficiency are examined. The couplings of the alignment parameters are then quantified in pairs and an assembly and alignment strategy is derived. This strategy is experimentally implemented in the assembly of two OPO test modules based on the new assembly technology with soldered optical mounts. Finally, the effects of thermal and mechanical loads on the optical output parameters are calculated and tolerance requirements for the optical mounts are derived from the results.
Livrozet, Marie Jeanne